stepping machanism of Apple Disk ][ drive (was Re: Heatkit 51/4 floppies)

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Sun Apr 11 11:48:28 1999

On Apr 11, 10:20, Richard Erlacher wrote:
> Subject: Re: stepping machanism of Apple Disk ][ drive (was Re: Heatkit
51
> I have to disagree with your comparison of the 2 MHz 6502 with a 4 MHz
> Z-80A. My thought here is that the 4MHz Z-80 used in the conventional
way,
> had a memory cycle of 750 nanoseconds (3 clock ticks), while the 6502, at
> whatever rate, again, used in the conventional way, had a memory cycle of
> one clock tick. Now, some instructions involve several memory cycles,
but
> that was true of both processor families. What I often cursed, was that
the
> textbook application of the 650x core left memory available (idle) half
the
> time. That was a blessing up to a point (2.5 MHz to be exact) because it
> allowed for DRAM "RAS-precharge." The Apple and others like it proved
that
> at around 1 MHz, the 6502's memory could be used for an entirely separate
> purpose, e.g. video refresh.

I wasn't talking about precisely 2MHz vs 4MHz, just a ballpark figure (as
opposed to "about the same" or "about ten times" clock speeds). So, given
the rest of your message, I think we're in broad agreement. BTW, BBC
Micros have a 2MHz clock on the 6502, and interleaved video and processor
access quite happily in 1980. The video took care of the refresh
requirement.

> I believe there are entirely too many subjective, architecture-related,
> factors to allow an absolute comparison/contrast of the two processors.

Agreed :-) That's why lies, damned lies, and benchmarks are so much fun
:-)

> In my "gut" I still believe the 4 MHz Z-80 is about
> comparable to a 1.5 MHz 6502.

Well, that's not very far from what I wrote, is it? I was just pointing
out that although Allison seemed to imply that a 6 or 8MHz Z80 was much
faster than a 4MHz(? I haven't got the original message any more) 6502, I
believe that to be far from the case.

-- 
Pete						Peter Turnbull
						Dept. of Computer Science
						University of York
Received on Sun Apr 11 1999 - 11:48:28 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:31:41 BST